Area Forecast Discussion
Issued by NWS Riverton, WY

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430
FXUS65 KRIW 170604
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1104 PM MST Mon Jan 16 2017

.SHORT TERM...TONIGHT THROUGH THURSDAY

SFC HIGH PRESSURE IS CONTROLLING THE AREA WX ATTM WITH THE CENTER
OF THE HIGH OVER WESTERN WY WHERE SOME OF THE COLDEST MORNING
TEMPS HAVE BEEN RECORDED WITH BONDURANT COMING IN AT 36 BELOW THIS
MORNING. LOWERED NIGHTTIME AND DAYTIME TEMPERATURES QUITE A BIT IN
THE VALLEYS AND BASINS THROUGH WED NT BASED ON THE TEMPERATURE
TRENDS DURING THE LAST COUPLE OF DAYS IN THESE AREAS. IN SPITE OF
THE WARMING ALOFT EXPECTED DURING THE NEXT COUPLE OF DAYS, WE ARE
STILL MANUFACTURING OUR OWN BRAND OF COLD AIR IN THE VALLEYS AND
BASINS. WITH EACH NIGHT THAT PASSES CONSISTING OF CALM WINDS AND
MOSTLY CLEAR SKIES, WE COOL THE LOWER LEVELS OF THE ATMOSPHERE A
COUPLE OF DEGREES AT A TIME THANKS TO THE SHORT DAYS AND LONG NIGHTS
AND LOW SUN ANGLE THIS TIME OF YEAR. THE WARM AIR ADVECTION ALOFT WILL
SIMPLY STRENGTHEN THE INVERSIONS. WITH A SIGNIFICANT SNOW COVER
STILL IN PLACE IN THE BASINS AND VALLEYS, AREAS OF NIGHT TIME AND
MORNING FOG ARE EXPECTED TO CONTINUE THROUGH WEDNESDAY MORNING.
BY WEDNESDAY NIGHT, THE UPPER LEVEL JET WILL SAG SOUTH INCREASING
OUR STEERING FLOW FROM THE SOUTHWEST AND SOUTH SOUTHWEST GIVING
WESTERN WYOMING AN INDIRECT FLOW OFF THE PACIFIC FOR LIGHT
SNOWFALL TO OCCUR MAINLY ON THURSDAY BEFORE THE ENERGY SPLITS AND
SHIFTS TO THE SOUTHWEST UNITED STATES. AS A SIDE NOTE, A SFC LEE
SIDE TROUGH WILL BE COINCIDENT WITH INCREASING SOUTHWEST WINDS
FROM SOUTH PASS TO CASPER BEGINNING TUESDAY. THESE MIXED AREAS AND
HIGHER ELEVATIONS WILL SEE A SIGNIFICANT WARMING TREND THROUGH
WEDNESDAY.

.LONG TERM...THURSDAY NIGHT THROUGH MONDAY

Mean negatively tilted trough remains just off the West Coast
through most of the period before a possible shift eastward
begins early next week. The first of several splitting shortwaves
moves into the west at the beginning of this period with some light
snow in the west and south before the southern piece heads towards
Srn CO. Another splitting trough on Saturday is even further south
with still some light snow in the west and far south but even less
than the Thursday night one. After some brief ridging Sunday, we see
another swinging through on Monday. This one may be a little more
potent as mean trough moves inland. Still a fast news moving
shortwave but enough to get a quick shot of decent snow in the west
and south and perhaps a little east of the divide. If nothing else,
these waves will bring in colder air aloft and help weaken our
inversions along with clouds to aid in warmer overnight lows.

&&

.AVIATION.../06Z ISSUANCE/

Warm air advection over a bitter cold surface air mass will
strengthen the surface inversion in basins and valleys. Where fog
eventually develops, it should be slower to much slower to
dissipate/lift due to a stronger surface inversion. A limiting
factor for fog development, and its thickness remains a marginal
amount of boundary layer moisture. KRKS and KRIW terminals followed
by KBPI have the best chance of fog development with a good chance
of airfield minimums occurring this morning. Elsewhere, confidence
is not as high so have limited MVFR/IFR conditions. Fog is not
expected at KCOD or KCPR with VFR conditions through the period.
Could see marginal LLWS on the lee side of the Absaroka
Mountains/Cody Foothills Tuesday morning and again Tuesday night,
but latest model guidance have trended weaker with a limited spatial
extent. Thus have decided to remove it from COD terminal for now.

&&

.FIRE WEATHER...

FIRE DANGER LOW TODAY ALONG WITH COLD TEMPERATURES, SIGNIFICANT
EXISTING SNOWPACK, AND RELATIVELY LIGHT TO MODERATE WINDS. A
STRONG COLD TEMPERATURE INVERSION WILL PERSIST THROUGH THE MIDDLE
OF THE WEEK WITH AREAS OF FOG EXPECTED WITHIN THE INVERSION ALONG
WITH POOR SMOKE DISPERSAL. SOUTHWEST WINDS WILL INCREASE FROM
SOUTH PASS TO CASPER BEGINNING TUESDAY WHERE BETTER MIXING WILL
OCCUR. IN FACT, THE SOUTHERN WIND CORRIDOR WILL LIKELY HAVE
STRONGER WINDS THAN THE HIGHEST MOUNTAIN PEAKS. LIGHT SNOW IS
EXPECTED IN WESTERN WYOMING ON WEDNESDAY.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
None.
&&

$$

SHORT TERM...Lipson
LONG TERM...Skrbac
AVIATION...Skrbac
FIRE WEATHER...Lipson



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